Superior heat transfer properties and light weight are desirable in heat exchangers, heat dissipaters, heat pipes, heat sinks, and the like, which are used in electronic devices, communication devices, and transportation devices such as automobiles and aircraft. While Cu (copper) has good heat transfer and heat diffusion properties, Al (aluminum) is widely used for heat exchangers and heat transfer materials because of its light weight and heat transfer properties that are almost as good as those of Cu. More specifically, improvements for enhancing heat transfer properties have been developed for Al heat exchangers used in electronic devices by enlarging the cooling area, increasing the material thickness, and the like. However, with the significant demands made in the electronic device field regarding compactness, thin design, light weight, and high performance, there is a limit to enhance the heat transfer properties obtainable from these improvements.
Such being the case, there is a demand for an Al—Cu bonded structure that combines the superior heat transfer properties of Cu with the light weight of Al to provide heat transfer properties that exceed those of Al while keeping weight increases at or below that of Cu.
Conventionally, many different methods for dissimilar metal bonding of Al and Cu have been investigated, e.g., solid-phase bonding methods such as friction welding and explosive bonding, and diffusion bonding. Some of these bonding methods have been implemented practically. With these bonding methods, however, the bonding of large areas or complex shapes is difficult, and there are restrictions on the shape and dimensions of component to be bonded whilst implementation for precision parts, e.g., electronic devices, is difficult.
Brazing is a technology that has been in wide use for a long time as a method for bonding metals. Because of its simplicity and the degree of freedom that is offers for the materials that can be used, the method can be easily applied to precision parts. Since it is expected that there will be a demand for lower-cost processing of precision parts, the application of brazing in bonding Al and Cu is being studied.
FIG. 1 shows a schematic illustration of a representative bonded interlayer where Al and Cu are directly brazed using an Al—Si—Mg—Bi-based brazing sheet. The brazing conditions in this case are as follows: an Al—Si—Mg—Bi-based brazing sheet is used; the brazing temperature is 803 K (530 deg C.), and the brazing time is 60 sec.
As the figure shows, the Al—Cu bond shows the formation of two intermetallic compounds: a layered δ phase and a θ phase of irregular form. Both of these are Al—Cu intermetallic compounds.
In order to clarify the properties of the Al—Cu bonded interlayer, the hardnesses of the Al base material, the Cu base material, and the intermetallic compound were measured. It was found that, while both based material Al and the Cu base material had a hardness of no more than Hv 100, intermetallic compounds (the layered δ phase and the θ phase of irregular form) had of Hv 480-620, which is considerably harder than base materials, implying brittle structure.
The results of a shear fracture test of the brazed joints indicated that the bond was brittle with regard to fractures mechanism while this was accompanied by almost no deformation in the base material. More specifically, deformation took place at the bonded interlayer, with the shear strength of the brazing joints being up to approximately 12.5 Mpa, while is considerably lower than the shear strength of 65 Mpa of the Al base material (industrial pure Al).
Thus, in the brazing joints where Al and Cu are directly bonded, the strength of the joints is determined by the strength of the intermetallic compound layer formned at the bonded interlayer. This results in a brittle fracture mechanism that limits the strength of brazing joints. For this reason, a structure in which Al and Cu are directly brazed cannot be used in the heat exchangers, heat dissipators, heat pipes, heat sinks, and the like described above.
Thus, at the bonded interlayer where Al and Cu are directly brazed, a δ phase and a θ phase are formed being made of Al and Cu, as two types of intermetallic compounds with extreme hardness, and their strengths and structural morphology affect the strength of the brazing joints.
In order to improve the strength of the Al—Cu brazing joints, brazing tests were performed using various metals as an insert material, and it was found that the strength of brazing joints could be best achieved when Ag is used as insert material. More specifically, by inserting Ag into the Al—Cu bonding interface, the creation of intermetallic compounds due to the direct reaction of Al and Cu could be restricted, thus improving bonding strength.
Based on observations such as this, the present inventors first proposed an invention relating to “a method and structure for bonding an Al component and a Cu component in which, when an Al component and a Cu component are bonded, a metal layer, more specifically an Ag layer, is formed on the bonding interface of the Cu component, and this Ag layer and the bonding interface of the Al component are brazed” (see Japanese patent application number 2002-321182).
FIG. 2 is a schematic illustration showing a representative structure of a dissimilar material bonded interlayer that uses an Al—Si brazing sheet, and that Ag is inserted into the Al—Cu bond interface as an insert material. The brazing conditions are as follows: an Al—Si—Mg—Bi-based brazing sheet is used; the brazing temperature is 823 K (550 deg C.), the brazing time is 600 sec.
According to an equilibrium diagram of the Ag—Cu binary system, this combination is a standard eutectic reaction system. Intermetallic compounds are not created over the entire composition range, and the eutectic temperature is as high as 1052 K (779 deg C.). As a result, the Cu—Ag interlayer does not exhibit structural changes, and there are no Al or Cu reactions or its derivative intermetallic compounds.
The zone where Ag-brazing sheet bonded interlayer shows complex morphology, and is divided into four regions, as shown in the schematic illustration in FIG. 2. Region I having an irregular form is generated on the reaction boundary between the brazing sheet and Ag, and Region II, formed as agglomerations, is found within this Region I. Region III is formed in Region IV as a lacy plate grown toward the brazing sheet side from Region I.
The results of X-ray analysis indicates that Region I and Region II are the Al—Ag intermetallic compounds Ag2Al. Also, Region II is Si in the brazing sheet, and Region IV is Al.
Testing the hardness of the intermetallic compound shows that Ag2Al has a hardness of approximately Hv 300, which is softer than the δ and θ phases shown in FIG. 1. A tensile test of the brazing joints shows ductile fracture of the Al base material, with the tensile strength of the brazing joints being similar to that of the Al base material. Thus, compared to the direct brazing of Al—Cu, the strength is significantly improved.
Thus, by using Ag as an insert material in an Al—Cu bonding interface and forming a brazing joints, the remained Ag obstructs direct reaction between Al and Cu. As a result, the formation of the harmful intermetallic compound can be prevented. Also, since A2Al, which is formed as an intermetallic compound, is relatively soft and is dispersed in a lacy manner in the Al matrix, the Ag—Al bonded interlayer shown in FIG. 2 is able to provide superior bonding characteristics.
When using Ag as an insert material in the Al—Cu bonding interface, however, the strength of brazing joints may not be stable depending on the brazing conditions. A brazing test was performed in which the brazing temperature was varied over the range of 793 K (520 deg C.)−843 K (570 deg C.) and the brazing time was varied over the range of 60-3600 sec. The brazing condition of test pieces under these conditions were visually inspected.
FIG. 3 shows the results from visual inspections of test pieces brazed using the above conditions with Ag used as an insert material. In this figure, “X” indicates poor brazing due to inadequate formation of a liquid phase; “Δ” indicates partial formation of a liquid phase; “◯” indicates good brazing with adequate liquid phase formation; and “□” indicates prominent fusing of the Al base material due to excessive liquid phase.
Based on the results shown in FIG. 3, there is a limited range of brazing conditions that provide good brazing. To providing consistent, stable bonding characteristics, it is important that the brazing temperature is set up in an appropriate range and an appropriate brazing time is set up.
As described above, the trend in the electronics device industry toward designs that are compact, thin, and light-weight and provide high performance leads to a demand for flexibility in design for various types of heat exchangers and heat transferring devices, since these can be used for precision parts.
The flexibility of design demanded of heat exchangers and heat transfer devices is not limited to dimensional accuracy of devices but also includes the flexibility in setting dimensions to accommodate the diversity in dimensions used by devices. Thus, with the Al—Cu bonded structure described above, there is a demand for superior flexibility in design.
As described above, in order to provide adequate strength of Al—Cu brazing joints, an innovative invention has been developed to provide an Al—Cu bonded structure and method for making the same in which Ag is used as an insert material. However, producing this Al—Cu bonded structure in a stable manner for wide use in heat exchangers and heat transfer devices requires solving a number of problems.
The first problem is that when performing Al—Cu brazing, stable bonding characteristics (tensile strength of brazing joints, deformation behavior) must always be maintained. The second problem is the need to provide superior dimensional accuracy to accommodate the dimension characteristics required in heat exchangers and heat transfer devices, and to provide flexibility in dimensions to accommodate the increasing diversity in dimensions.